This invention relates generally to machines for screening a particulate, granular or other multi-sized material and treating a size fraction thereof. More particularly, this invention pertains to portable apparatus useful for soil remediation and the like.
At numerous locations, the ground has been contaminated by accidental or deliberate application of gasoline, fuel oil, used petroleum products, or various chemicals. In most soil remediation processes, the contaminated soil is first removed from the ground and separately treated to chemically or biochemically remove the contaminant or transform it into a non-soluble, non-volatile and/or non-offensive material. In most of these processes, a remediating additive is to be introduced and mixed with the contaminated soil.
In one type of process, specialized microorganisms which consume the contaminant may be introduced or dosed in either solid or a liquid form.
In other remediating processes, various chemical entities may be added which bind with or encapsulate the contaminant. Certain contaminants may be chemically converted to non-toxic or less toxic substances.
In most cases, the soil must be pulverized into small particles so that material surfaces are effectively exposed to the additive. In addition, the contaminated soil and additive must be intimately mixed to ensure rapid remediation.
The extent of many contaminated sites requires large quantities of soil to be removed and treated. For example, a 5 acre site contaminated to a depth of 6 feet requires the removal of about 50,000 cubic yards of material weighing about 40,000 to 100,000 tons, depending upon the type of soil and moisture content. Economic treatment typically dictates high treatment rates of 50 or more tons per hour. Thus, a treatment plant operated at 80 tons per hour will take about 5 months (at 168 hours/month up-time) to complete the remediation operation.
Many contamination sites are much smaller, of course, requiring much less treatment time. Likewise, some contamination sites are of considerably greater extent.
In the past, equipment for performing such large-scale remediation was mounted on a plurality of trailers or trucks, each separately transported to the remediation site. Multiple trailers requiring intermediate process equipment results in higher equipment costs. Significant costs associated with such operations include the use of multiple truck drivers. In addition, if oversize trailers are used, escort vehicles may be required, increasing the overall cost of travel between operation sites.
The plurality of trailer mounted units also requires considerable on-site assembly time for connecting the trailers with conveyors to form a single operating train. Additional conveyors may be required for transporting the process material from trailer to trailer, and the plant may require more ground area than is desirable, i.e. have an excessive "footprint".
One of the factors which must be addressed in designing a treatment train is the change in elevation of the material being treated as it passes through a treatment step. When the elevation drop is great enough so that the material cannot be discharged from one treatment step directly into the following step, a conveyor must be used to gain the desired elevation. Because of the sizeable elevation drop of the "fines" through a screening operation, portable apparatus for screening have required an intermediate hopper and conveyor for collecting screened material and feeding it to a mixer.
A related constraint affects the sizes of the belt conveyors. The angle at which solids may be conveyed upwardly by belt conveyor has a relatively low maximum value. If this value is exceeded, the materials being conveyed may slip backwards and/or fall off the sides of the conveyor belt. At a given angle, the minimum length of a conveyor is proportional to the vertical difference between the inlet and outlet elevations.